Frequency sharing between satellite systems and terrestrial fixed service (FS) systems has been object of studies since the onset of commercial satellite communication systems. A particular case of interest refers to the interference produced by the downlink of highly elliptical orbit satellite systems (HEO) into fixed service receivers. In the literature, studies involving this type of interference have presented analyzes that consider the simplifying assumption that all victim fixed service receivers have their receiving antennas with zero degree elevation angle. In this work a probabilistic analysis is used to evaluate the aggregate interference produced by the satellites of multiple highly elliptical orbit satellite systems into fixed service receivers operating in the 18 GHz frequency band. The FS receiving antenna elevation angle is modeled as a random variable with known probability density function. The proposed mathematical model is applied to two scenarios involving multiple interfering HEO systems. More specifically, the first scenario considers three interfering HEO systems having satellites that operate only in the northern hemisphere. In the second scenario, three HEO systems with satellites that operate in both the northern and southern hemisphere are considered. The obtained results are compared to those resulting from analyses that use the hypothesis that the receiving antennas of all the FS victim systems have a zero degree elevation angle.